We have in the past year developed a new technique "label-fracture" which allow the observation of the distribution of a cytochemical label of antigens and receptors on cell surfaces. Cells are fixed in glutaraldehyde and labeled with an electron dense marker (colloidal gold). They are then frozen, freeze-fractured and replicated by platinum/carbon evaporation. The exoplasmic halves of the membrane, stabilized by the deposition of the Pt/C replica, are washed in distilled water. Mounted on formvar coated grids and then examined on an electron microscope. This new technique reveals the surface distribution of the label coincident with the Pt/C replica of the exoplasmic fractured face. We are now applying this technique to study the cell surface microdomains of sperm and insulin binding sites on the surface of animal fat cells. "Label-fracture" has extraordinary resolution (avg. less than 5nm) and will allow mapping at the supramolecular level of receptors and antigens on cell surfaces while relating directly to the freeze-fracture morphology of the plasma membrane.